linux-kernel - [PATCH AUTOSEL 6.13 34/34] ring-buffer: Make reading page consistent with the code logic

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Message-Id: <20250126145310.926311-34-sashal@kernel.org>
Date: Sun, 26 Jan 2025 09:53:10 -0500
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Jeongjun Park <aha310510@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sasha Levin <sashal@...nel.org>,
	mhiramat@...nel.org,
	linux-trace-kernel@...r.kernel.org
Subject: [PATCH AUTOSEL 6.13 34/34] ring-buffer: Make reading page consistent with the code logic

From: Jeongjun Park <aha310510@...il.com>

[ Upstream commit 6e31b759b076eebb4184117234f0c4eb9e4bc460 ]

In the loop of __rb_map_vma(), the 's' variable is calculated from the
same logic that nr_pages is and they both come from nr_subbufs. But the
relationship is not obvious and there's a WARN_ON_ONCE() around the 's'
variable to make sure it never becomes equal to nr_subbufs within the
loop. If that happens, then the code is buggy and needs to be fixed.

The 'page' variable is calculated from cpu_buffer->subbuf_ids[s] which is
an array of 'nr_subbufs' entries. If the code becomes buggy and 's'
becomes equal to or greater than 'nr_subbufs' then this will be an out of
bounds hit before the WARN_ON() is triggered and the code exiting safely.

Make the 'page' initialization consistent with the code logic and assign
it after the out of bounds check.

Link: https://lore.kernel.org/20250110162612.13983-1-aha310510@gmail.com
Signed-off-by: Jeongjun Park <aha310510@...il.com>
[ sdr: rewrote change log ]
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/trace/ring_buffer.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 60210fb5b2110..6804ab126802b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -7059,7 +7059,7 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 	}

 	while (p < nr_pages) {
-		struct page *page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+		struct page *page;
 		int off = 0;

 		if (WARN_ON_ONCE(s >= nr_subbufs)) {
@@ -7067,6 +7067,8 @@ static int __rb_map_vma(struct ring_buffer_per_cpu *cpu_buffer,
 			goto out;
 		}

+		page = virt_to_page((void *)cpu_buffer->subbuf_ids[s]);
+
 		for (; off < (1 << (subbuf_order)); off++, page++) {
 			if (p >= nr_pages)
 				break;
-- 
2.39.5